[Paper Review] Effect of Chemical Composition on Enthalpy of Evaporation and Equilibrium Vapor Pressure

Vladimir Kh. Dobruskin|arXiv (Cornell University)|Apr 20, 2010

Advanced Thermodynamics and Statistical Mechanics6 references4 citations

TL;DR

This paper derives a correlation between partial enthalpy of evaporation, component concentrations, and equilibrium vapor pressure in binary solutions using the Clausius-Clapeyron equation. It introduces a three-stage model showing that surface layer energetics dominate the enthalpy of evaporation, with adsorption-induced surface composition changes explaining differences in D(ΔH) vs. X curves between ideal and non-ideal solutions.

ABSTRACT

Proceeding from the Clausius-Clapeyron equation, the relation is derived that establishes a correlation between the partial enthalpy of evaporation from binary solutions, concentrations of components, and equilibrium vapor pressures. The difference between enthalpies of evaporation of components from solutions and those from the pure liquids, D(DH), depends on the chemical nature and concentrations, X, of solutions. The effect of concentrations on D(DH) makes different appearances in ideal and non-ideal solutions, although, as a whole, D(DH) increases with the growth of concentration of the second component. A model is introduced, which considers D(DH) as the sum of energetic changes of three sequential stages: passage of molecules from the bulk liquid into the surface layer, exit of the molecules on the outer side of the interface, and the following desorption into the gas phase. In the framework of the model, the main contribution to enthalpy of evaporation comes from the processes in the surface layer. It is suggested that adsorption from solutions, which changes the chemical composition of the surface layer with respect to that of the bulk solution, determines, to great extent, the difference in the forms of the curves D(DH)=f(X) for ideal and non-ideal solutions.

Motivation & Objective

• To establish a quantitative relationship between partial enthalpy of evaporation, component concentrations, and equilibrium vapor pressure in binary solutions.
• To explain the deviation of enthalpy of evaporation from pure liquid values (Δ(ΔH)) as a function of solution composition and chemical nature.
• To clarify why Δ(ΔH) exhibits different concentration dependencies in ideal versus non-ideal solutions.
• To develop a mechanistic model for the enthalpy of evaporation based on sequential interfacial processes.
• To identify surface composition changes due to adsorption as the key factor governing the shape of Δ(ΔH) vs. X curves.

Proposed method

• Derives a correlation from the Clausius-Clapeyron equation linking partial enthalpy of evaporation, vapor pressure, and mole fractions.
• Proposes a three-stage model: (1) transfer from bulk liquid to surface layer, (2) migration to the outer interface, and (3) desorption into gas phase.
• Models Δ(ΔH) as the sum of energetic contributions from each stage, with the surface layer process contributing most significantly.
• Introduces the concept that surface composition, altered by selective adsorption, determines the form of Δ(ΔH) vs. X dependence.
• Distinguishes between ideal and non-ideal solutions based on how concentration affects Δ(ΔH), attributing differences to surface-specific interactions.
• Uses thermodynamic analysis to relate macroscopic vapor pressure and enthalpy data to microscopic interfacial processes.

Experimental results

Research questions

• RQ1 How does the chemical composition of a binary solution influence the partial enthalpy of evaporation and equilibrium vapor pressure?
• RQ2 What causes the difference in Δ(ΔH) behavior between ideal and non-ideal solutions as a function of concentration?
• RQ3 Which interfacial process—bulk-to-surface transfer, surface migration, or desorption—contributes most to the enthalpy of evaporation?
• RQ4 To what extent does surface composition, altered by adsorption, govern the shape of the Δ(ΔH) vs. X curve?
• RQ5 How can the Clausius-Clapeyron equation be extended to describe partial enthalpies in non-ideal solutions with surface-specific effects?

Key findings

• The difference in enthalpy of evaporation from solutions versus pure liquids, Δ(ΔH), increases with the concentration of the second component in the solution.
• The surface layer processes contribute the largest share to the total enthalpy of evaporation in the proposed three-stage model.
• Adsorption from the solution alters the surface composition relative to the bulk, which is the primary factor explaining the distinct shapes of Δ(ΔH) vs. X curves in ideal and non-ideal systems.
• In non-ideal solutions, the concentration dependence of Δ(ΔH) deviates significantly from ideal behavior due to surface-specific interactions and adsorption effects.
• The model successfully accounts for the observed variation in Δ(ΔH) with composition by attributing it to interfacial energy changes during evaporation.
• The derived correlation between vapor pressure, concentration, and partial enthalpy of evaporation is consistent with thermodynamic principles and provides a predictive framework for solution behavior.